The temporal evolution of microstructures during structural transition of D0₂₂ and L1₂ involved with transient phases

A microscopic phase-field method was employed to study the evolution in microstructure of Ni-Al-V alloys during the ordering process. The FCC → L1₂/D0₂₂ phase transition proceeds with an intermediate transient L1₀/L1₀ (M = 1) phase at the critical Al level where the two phases co-exist. Additionally, L1₂ may act as a transient phase of D0₂₂ at compositions not in favor of D0 ₂₂, and vice versa. The rearrangement of atoms during the structural transition undergoes three classical stages: a respective disorder stage with atoms distributing uniformly; a second transient phase with atoms aligning in either L1₀/L1₀ (M = 1) configuration with slight atom fluctuation, or in L1₂-Ni₃Al/D0₂₂-Ni ₃V configuration, with atoms highly aligned, and lastly; a third equilibrium L1₂-Ni₃Al/D0₂₂-Ni₃V phase. Further studies on the atomic occupancy probability demonstrated that L1₀ and L1₀ (M = 1) are low-ordered phases with atomic occupancy probability fluctuation within a narrow range; conversely, L1 ₂ and D0₂₂ are ordered phases showing significant atomic occupancy probability fluctuations when ordering as well as when act as a transient phase.